This invention relates generally to residential furnaces and, more particularly, to a method and apparatus for temporarily operating a furnace from an auxiliary power source.
The primary energy source for most residential furnaces is a fossil fuel such as gas or oil. However, electrical power is also used in such furnaces in order to provide power for the control systems and for the auxiliary equipment such as blowers and the like. Thus, if electrical power to a home is interrupted such as sometimes occurs during ice storms and the like, a home may be left without heat for hours or even days at a time. On these occasions, it would be desirable to operate the furnace on an emergency power basis.
The usual types of auxiliary power used for emergency purposes include small internal combustion engine driven emergency power generators, or a bank of 12V DC batteries operating through a DC/AC power converter. In neither case would there be sufficient power over the necessary time in order to sustain continued operation of the standard furnace. The primary reason is that a standard furnace will draw too much power, such that the auxiliary source cannot maintain continued operation thereof over a prolonged period of time.
Conventional furnaces with 60,000 BTU heating output generally consume 500-800 watts of electrical power when operating. If a 12V DC battery is used to provide this power through a power converter, the furnace would draw 40-67 amps from the battery. Such a high current would drain a single marine battery very quickly. Accordingly, it would be necessary to use several batteries in parallel in order to deliver the necessary current over a reasonable operating time.
Considering now the possibility of operating the furnace with a standard 100-200 watt power converter powered from an automobile battery, if the automobile engine is allowed to idle at 500-600 rpm, the alternator will produce only about 10-20 amps. In order for the alternator to deliver the required current for proper operation of the furnace, it would be necessary to increase the engine speed to 2,000 rpms, which in turn would require someone to sit in the car and hold the throttle pedal down or to readjust the idle screw on the carburetor. Similarly, in the case of a small emergency generator driven by an internal combustion engine, it would be necessary to have a substantially large generator or a substantially large internal combustion engine in order to obtain the desired current level to sustain operation of the furnace.
It is now common practice to provide dual rate burners in furnaces such that they can operate in either the low or the high heat mode depending on the heating load. In addition to the draft inducer motor operating in the low and high speed modes, the air circulation blower is also selectively run at either low or high speeds, corresponding to the respective low or high mode of the burner. While the ambient temperature conditions and the character of the space to be heated will determine the relative mix of high and low fire operation, there will normally be some time periods in which the system will be caused to operate in a high fire mode. When the ambient conditions are such that power failures are caused to occur (e.g., ice storms), then the likelihood of operating in a high fire mode increases. It is this mode, and in particular the circulation blower, that tends to draw more current and to thereby overload the auxiliary power system as discussed hereinabove. That is, the applicants have recognized that the power draw of the circulation blower motor operating in the high heat mode is on the order of six times that for the same motor operating in the low heat mode.
One of the times when the furnace normally operates its circulation blower in a high speed mode is when there is a temporary power loss followed by a recovery of power immediately thereafter. If the system had been operating at the time, there would still be a call for heat but the heat exchanger would still be hot. In this situation, in order to prevent the tripping of the limit switch because of the heat exchanger becoming too hot upon resumption of the heating cycle, the circulation blower is normally caused to be automatically turned on at high speed in order to cool off the heat exchanger and thereby avoid the tripping of the limit switch. This occurrence would, of course, tend to draw down the current of any auxiliary power source that may be brought on line after a power interruption.
It is therefore an object of the present invention to provide an improved method and apparatus for operating a furnace from an auxiliary power source.
Another object of the present invention is the provision for operating a furnace from an auxiliary power source without drawing excessive current from the source.
Yet another object of the present invention is the provision in a dual rate furnace for operating for prolonged periods with an auxiliary power source.
Still another object of the present invention is the provision for temporarily operating a furnace from an auxiliary power source in an effective and economical manner.
These objects and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.